For me personally, I found the iPhone battery just didn't have the power to get me through the day if I used it to its full potential (I deal with a lot of e-mails during the day on my mobile device). For that reason, I've now got a separate cell phone and a 32GB iPod touch. I find splitting the work between the two devices suits me better.
However, if you are a lighter user then the iPhone is a fantastic device and does have the advantage that you only need to carry one thing around with you.
Ultimately, only you can make the decision which option suits your personal circumstances the best, but either device will serve you well